drm/i915/mpllb: use I915_STATE_WARN() for state mismatch warnings

The pipe_config_mismatch() function is primarily for logging comparison
results.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/167e54e13a9a41c944910a274e79cbfd39d963b1.1655372759.git.jani.nikula@intel.com